The present invention relates to a lithium secondary cell and a method of manufacturing thereof, and more particularly to a lithium secondary cell in which a high power density is required and a method of manufacturing thereof.
In recent years, cylindrical lithium ion secondary cells have been expected as a secondary cell that can achieve a large capacity and a high power density.
In such lithium secondary cells, the following drawbacks have been noted, particularly when such cells are required to have a large capacity and a high power density.
(1) The length of a sheet type electrode plate inevitably becomes large, and therefore, if the number of tabs is small, an effect of current collection declines. As a result, an internal resistance of the cell becomes large, and the cell performance is consequently deteriorated.
(2) In order to avoid unevenness of the potential distribution on the electrodes, it is required that the current collector tabs be evenly arranged in the longitudinal direction of the current collector. As a consequence, in order to avoid unevenness of reaction, it is required that each of the positive electrode active material and the negative electrode active material be present in a well-balanced manner on each opposite surface in each of the electrodes, with a separator interposed between the electrodes. It is to be noted here that the term the xe2x80x9cunevenness of reactionxe2x80x9d is intended to mean that when a negative electrode active material does not exist in a position in a negative electrode plate corresponding to a positive electrode active material, metal lithium is deposited because of the inhibition of the intercalation of lithium ions in the corresponding position.
In order to provide a solution for the above drawbacks, several types of lithium secondary cells have been suggested. One typical example of such a prior art cell is shown in a Japanese Unexamined Patent Publication No. 6-267528, which discloses a lithium secondary cell having the following construction. The cell has a sheet type current collector for both the positive and the negative electrodes, and in both electrodes, one lengthwise side part (i.e., both the upper and bottom portions of the spirally wound electrode assembly) is protruded from a separator with no active material applied thereon, so as to form a positive and a negative electrode lead connecting parts. A plurality of positive electrode leads are then connected to the positive electrode connecting parts, and a plurality of negative electrode leads are likewise connected to the negative electrode connecting parts.
The above prior art cell, however, has such drawbacks that, since the current collector tabs are welded to only one end of the current collector, the potential variation in the width direction of the electrodes is rendered large, that a large output power is not possible, and that a cycle life of the cell becomes short.
In view of the above-described drawbacks in prior art, it is an object of the present invention to provide a lithium secondary cell in which imbalance between the capacities of the positive electrode and the negative electrode is minimized, potential variation is reduced, and a high output power is achieved. The characteristics of the present invention are detailed in the following.
According to the present invention, there is provided a lithium secondary cell comprising a spirally-wound electrode assembly in which a positive electrode plate and a negative electrode plate are spirally wound with a sheet type separator interposed therebetween, the positive electrode plate comprising a sheet type positive electrode current collector and a positive electrode active material formed on both surfaces of the positive electrode current collector, and the negative electrode plate comprising a sheet type negative electrode current collector and a negative electrode active material formed on both surfaces of the negative electrode current collector, the lithium secondary cell characterized in that:
a plurality of positive electrode current collector tabs are attached to the positive electrode plate so that an interval is formed between the plurality of the positive electrode current collector tabs in a longitudinal direction of the positive electrode plate;
a plurality of negative electrode current collector tabs are attached to the negative electrode plate so that an interval is formed between the plurality of the negative electrode current collector tabs in a longitudinal direction of the negative electrode plate; and
in a state of the positive electrode plate and the negative electrode plate being wound, each of positive electrode current collector tabs and each of the negative electrode current collector tabs are so disposed as to face each other with the separator interposed therebetween.
According to the above construction, since the plurality of positive electrode current collector tabs and the plurality of the negative electrode current collector tabs are disposed so as to face each other with the separator interposed therebetween, the negative electrode active material is made to be present in a position corresponding to the positive electrode active material. As a result, deposition of metal lithium can be avoided, and a charge-discharge cycle life of the cell is improved. In addition, since the positive electrode current collector tabs and negative electrode current collector tabs are disposed so as to face each other in the longitudinal direction, unevenness of potential distribution caused in the longitudinal direction of the positive and the negative electrode plates can be thereby suppressed, and a cell with a high output power can be achieved.
Moreover, since the current collector tabs are provided in plurality in the longitudinal direction of the positive and the negative electrode plates, efficiency of current collection is thereby increased, and as a result, it is made possible to reduce an internal resistance of the cell and thereby improve the cell performance.
In addition, in the above construction of the lithium secondary cell, the following construction may be further employed. In the positive electrode plate, an applied area on which a positive electrode active material is applied and a non-applied area on which the positive electrode active material is not applied, are formed on the positive electrode plate alternately in the longitudinal direction of the positive electrode plate;
the positive electrode current collector tabs are firmly attached to the positive electrode current collector within non-applied area; and
a ratio W (L4/L2) of a width L4 of the non-applied area to a width L2 of the negative electrode current collector tab is between 1.2 and 3.5.
According to the above construction, because the aforementioned positive electrode current collector tabs are firmly fixed to the positive electrode current collector, it is made possible to realize a state in which the negative electrode current collector tabs are opposed to the positive electrode current collector tabs within the non-applied area in the electrode assembly being wounded together, even if a little displacement of the positive and the negative electrode plates occurs when winding the plates together. Accordingly, such an undesirable result is avoided that the positive electrode active material applied area and the negative electrode tab are overlapped, and therefore deposition of metal lithium is prevented.
The reason for restricting the ratio W to be between 1.2 and 3.5 are as follows. Firstly, if the ratio W is less than 1.2, there is a possibility of the negative electrode current collector tabs being overlapped with the positive electrode active material. On the other hand, if the ratio W is more than 3.5, the utilization factor of the positive electrode active material is reduced and thereby a high output power cannot be obtained.
In addition to the above-described constructions, the interval between the positive electrode current collector tabs and the interval between the negative electrode current collector tabs may be rendered to be within the range of 20 cm to 80 cm.
In the case where an increased number of the current collector tabs is required depending on the output power of the cell, the number of tabs can be increased by narrowing the interval of the current collector tabs. In such a case, if the interval of the tabs are made to be less than 20 cm, the winding of the positive and the negative electrode plates is rendered difficult since the number of the tabs becomes too large. Therefore, the interval of the tabs should be made at least 20 cm. On the other hand, if the interval of the tabs exceeds 80 cm, unevenness of potential distribution is incurred since the number of the tabs is too small, and therefore, the interval of the tabs is preferred to be 80 cm or smaller.
In addition to the above constructions, a width of the positive electrode current collector tab and a width of the negative electrode current collector tab may be both within the range of 10 mm to 30 mm.
The reasons for restricting the widths of the tabs are as follows. If a width of the tabs is made to be less than 10 mm, an allowable current value is rendered small, and consequently the number of the tabs needs to be increased. However, if the number of the tabs are increased, winding the positive and negative electrode plates becomes difficult. On the other hand, if the width of the tabs exceeds 30 mm, winding the positive and negative electrode plates is also rendered difficult since the width of the tabs becomes too large.
Further, according to the present invention, there is provided a method of manufacturing a lithium secondary cell comprising the steps of:
producing a positive electrode plate by forming a positive electrode active material layer on both surfaces of a sheet type positive electrode current collector, and thereafter firmly attaching a plurality of positive electrode current collector tabs so that an interval is formed between the plurality of positive electrode current collector tabs in a longitudinal direction of the positive electrode plate;
producing a negative electrode plate by forming a negative electrode active material layer on both surfaces of a sheet type negative electrode current collector; and
spirally winding the positive electrode plate, the negative electrode plate, and the separator by firmly attaching a plurality of negative electrode current collector tabs on the negative electrode plate so that each of the plurality of negative electrode current collector tabs is opposed to each of the plurality of positive electrode current collector tabs, with interposing a sheet type separator between the positive electrode plate and the negative electrode plate.
According to the above method, it is made possible to readily manufacture a lithium secondary cell having the constructions as described above.